Conventional cameras produce a synchronization signal known as an “X-sync” signal. An X-sync signal is initiated when a first shutter of the camera moves to a fully open position during an image acquisition. In one example, a mechanical sensor detects the shutter blade coming to a stop in motion. An X-sync signal can be used to fire a flash device to emit light during an image acquisition. As discussed further below, cameras typically have a maximum shutter speed (e.g., “faster” shutter speed that correlates to a shorter opening of the shutter) at which synchronization using X-sync can occur without “clipping” occurring in the image. This shutter speed defines the maximum X-sync for a given camera. Clipping is when flash lighting illuminates the imaging sensor (or alternatively film) unevenly due to light emission during a shutter blade traveling across the sensor. Clipping appears as a band of darker exposure in the image (e.g., at the top or bottom of the image).
FIGS. 1 and 2 illustrate timing plots related to one example of a conventional photographic flash synchronization system and method for an exemplary camera having a two blade focal plane shutter system. In this example of FIG. 1, the shutter speed is set at a relatively slower shutter speed setting (i.e., having a longer opening of the shutter) than the example discussed below with respect to FIG. 2. FIG. 1 includes a timing plot 105 showing mirror movement from an initial closed position to an open position (i.e., a position blocking the light path from the camera lens from the shutter mechanism to a position that allows light to pass to the shutter mechanism). FIG. 2 includes a timing plot 205 showing mirror movement from an initial closed position to an open position. Timing plot 110 of FIG. 1 and timing plot 210 of FIG. 2 each show movement of the edge of the first shutter blade to travel across an imaging sensor of the camera in the respective examples to a position in which the first shutter blade allows light to pass to the entire imaging sensor. Timing plot 115 of FIG. 1 and timing plot 215 of FIG. 2 each show movement of the edge of the second shutter blade to travel across the imaging sensor in the respective examples to a position that blocks all light from passing to the imaging sensor. In each of plots 105, 110, 115, 205, 210, 215, the lower horizontal line of the plot represents a fixed position prior to movement, the upper horizontal line of the plot represents a fixed position after movement, and the slanted line there between represents the time of movement.
In the example of FIG. 1, the time 127 between vertical dashed line 120 and vertical dashed line 125 is the time in which both shutter blades are in the fixed open positions allowing light to travel from the camera lens to the imaging sensor of the camera. In this example, during the time between lines 120 and 125 the first and second shutter blades do not obstruct the light to the sensor. In some examples, the first shutter blade of a camera will start movement at a time prior to beginning to allow light to pass to the imaging sensor (i.e., the starting position of the first shutter blade is at a distance from the edge of the imaging sensor) and the first shutter blade fully stops obstructing light from passing to the imaging sensor at a time prior to the first shutter blade stopping movement (e.g., at time 120). A camera may have a distance between the edge of the imaging sensor and the location where the shutter blade comes to a stop (e.g., to prevent damage to the shutter blade due to an instantaneous abrupt stop). Likewise, the second shutter blade of a camera may start movement at a position that is a distance from the edge of the imaging sensor such that it does not start to block light from passing to the imaging sensor until a time after the second shutter blade begins movement (e.g., at time 125) and the second shutter blade fully blocks light from the imaging sensor at a time prior to stopping movement. Dashed lines 130 and 230 mark the time at which the second shutter blade in each example, respectively, stops movement.
In the example of FIG. 2, the time 227 between vertical dashed line 220 and vertical dashed line 225 is the time in which both shutter blades are in the fixed open positions allowing light to travel from the camera lens to the imaging sensor of the camera. In this example, during the time between lines 220 and 225 the first and second shutter blades do not obstruct the light to the sensor.
The time between the first shutter blade of a camera stopping movement and the second shutter blade stopping movement (shown in the example of FIG. 1 as time period 135 and in the example of FIG. 2 as time period 235) may be referred to as the exposure time and is typically measured as the shutter speed of the camera. Plots 140 and 240 show a conventional synchronization signal (commonly referred to as a “synch” signal or an X-Sync signal) of the examples of FIGS. 1 and 2, respectively. Synch signals 140 and 240 are indicated by a voltage change at time 120 and 220, respectively, and a return to prior voltage at time 130 and 230, respectively. A conventional synch signal begins when the first shutter blade stops movement. In one example, a sensor in the camera detects the first shutter blade coming to a stop and causes an electrical signal that initiates an X-sync signal. In one such example, there may be some additional movement of the first shutter blade after the activation of the sensor (e.g., due to the actuation of a mechanical element of the sensor, due to bounce of the blade from the force of slapping open). Such movement after the normal temporal location for the activation of the X-sync signal of a camera is not included in the time determination for the stopping of the movement of the first shutter blade.
Plot 145 shows a plot of light emission over time from a photographic lighting device associated with the camera of the example of FIG. 1. Horizontal dashed line 150 marks the critical level above which the light emission of the lighting device is detectable by the imaging sensor of the camera over ambient light. The hatched area under the curve of the light emission profile represents light emission that contributes to the imaging by the camera sensor. Plot 245 shows a plot of light emission over time from a photographic lighting device associated with the camera of the example of FIG. 2. Horizontal dashed line 250 marks the critical level above which the light emission of the lighting device is detectable by the imaging sensor of the camera over ambient light. The hatched area under the curve of the light emission profile represents light emission that can contribute to the imaging by the camera sensor. Light emission is initiated in response to the synch signal. In the examples of FIGS. 1 and 2, a slight delay is shown between the sync signal and the initiation of light emission by the lighting device (e.g., possibly due to circuitry delay in the lighting device and/or time required to wirelessly transmit a light emission initiation signal to a lighting device that is remote from the camera).
The entire area above line 150 falls between line 120 and line 125 during the time period 127 in which the first and second shutter blades are not moving and the sensor is fully unobstructed by the two shutter blades. Thus, the light emission from the photographic lighting device in the example of FIG. 1 with the relatively longer shutter speed does not contribute to imaging during the time when the shutter blades are traveling across the imaging sensor. This is not true for the example of FIG. 2 with the faster shutter speed. A significant amount of the detectable light emission of the lighting device of plot 245 occurs after the second shutter blade begins movement and obstruction of the imaging sensor. This may cause uneven lighting of different portions of the imaging sensor and cause uneven darkening areas of the resultant image (e.g., referred to as “clipping”). Due to this limitation of the conventional synchronization method, photography with flash lighting is typically limited to shutter speeds that are slower (i.e., longer) than a particular shutter speed. For example, many cameras cannot adequately synchronize flash lighting at shutter speeds greater than 1/200th of a second.
One way to allow for shorter shutter speeds includes utilizing rapidly pulsed light bursts of a lighting device to produce a pseudo-continuous light source with a duration that spans from before initial shutter blade movement to well after final shutter blade movement. Such a system utilizes a great deal of extraneous energy before and after the actual image acquisition time period. This may result in excess depletion of lighting power sources. This type of synchronization is often referred to as “FP-sync.” It is also known in certain cameras manufactured by Canon as HSS, HS-sync, and/or “high-speed” sync. Herein, this type of synchronization is referred to as “FP-sync” and/or “FP-type sync.” FIG. 3 illustrates timing plots associated with one such example of an FP-type sync process. Plot 310 shows the movement of the first shutter blade of a camera similar to plots 110 and 210 discussed above. Plot 315 shows the movement of the second shutter blade of a camera similar to plots 115 and 215 discussed above. Dashed line 320 marks the time of the first shutter blade stopping movement. Dashed line 325 marks the time of the second shutter blade starting movement. The time 327 between lines 320 and 325 marks the time period in which the first and second shutter blades are not moving and are in the fully open position allowing light to pass to the imaging sensor of the camera. Dashed line 330 marks the time of the second shutter blade stopping movement after the edge of the second shutter blade has traveled across the imaging sensor. The time 335 between lines 320 and 330 represents the shutter speed. Plot 340 shows a conventional sync signal as a voltage change starting at time 320 to time 330. Plot 345 shows a photographic light emission profile intensity curve. Dotted line 350 indicates the start of movement of the first shutter blade. Light emission begins at a time prior to the first shutter blade beginning movement. The light emission reaches a peak and the lighting device is rapidly pulsed such that a pseudo-continuous light emission level begins prior to the first shutter blade beginning movement. This light emission must is held at this level until a time after line 330 (i.e., after the second shutter blade fully obstructs light from passing to the imaging sensor). This ensures a near constant light emission above ambient light during all times that the imaging sensor is either partially or fully unobstructed by the shutter blades. However, plot 345 shows significant light emission over an extended period of time. Such light emission may utilize a great amount of energy and possibly deplete lighting device power supplies.